Interleukin-4 (IL-4) has been demonstrated to be an important lymphokine for the generation of cytotoxic T lymphocytes (CTLs). Here we describe an IL-4 inducible gene specifically expressed in CTLs. By sequence homology, this gene is likely to be the mouse homolog of pancreatic lipase. Oocyte translation of in vitro transcribed mRNA results in the expression of a protein with lipase activity, and Northern analysis of various tissues and a large panel of hematopoietic cell types demonstrates that this gene is expressed only in the pancreas and CTLs. Lysates of CTLs grown in IL-4, but not in IL-2, exhibit lipase activity. Furthermore, Northern analysis of CTLs grown in the presence of IL-4 for as little as 5 days demonstrates a marked induction of lipase mRNA, which correlates with enhanced cytolysis by these cells. These results suggest that this lipase may have an important role in CTL effector function.